1. Field of the Invention (Technical Field)
The present invention relates to extended optimal filtering methods, systems, apparatuses, and computer software for adaptive on-transmit pseudorandom noise radar waveforms. Embodiments of the present invention thus simultaneously provide low sidelobe level and spectral purity without degrading the main peak of the auto-correlation function.
2. Description of Related Art
In radar applications, appropriate transmit waveforms are of vital importance for target detection, non-ambiguous estimation of range and range-rate, accuracy, resolution, and clutter rejection. Therefore, the radar designer has to carefully examine and choose the transmit waveforms to achieve the desired objectives of the intended system. In general, the optimal design involving both hardware and software components usually perform optimally under specific operational conditions for which the system was designed. The performance of such a system may degrade if the operational environment undergoes an adverse change. Examples of such environmental degradation could include, among others, increasing noise level, changing propagation channel, jamming or interference(s), the nature of the targets, and so on. The radar system should, therefore, be capable of dynamically adjusting system parameters to optimize its performance. Adaptive techniques such as antenna beamforming or space-time adaptive processing, mainly at the receiver end, have been tried to mitigate these effects.
At the transmitter end, the adaptive-on-transmit (AT) methodologies based on the waveform design constitute the biggest family of AT approaches. The concept of AT is not new, having been considered at various stages in the past. Technological advances over the last couple of decades in generating and manipulating digital waveforms have provided further impetus to real-time AT waveforms. AT methodologies can be further subdivided into waveform selection and waveform design. In the first case, the parameters of the transmitted signal can be adaptively selected from a predefined set. In the second case, the parameters are dynamically estimated according to the changing operational environment, thereby demanding large computational power. Both AT techniques, selection or design, are further constrained by the system hardware limitations. The trade-off between the system capabilities and the waveform-based AT approach makes it much harder to design an optimal performing system under all operational conditions.
Waveform-filter pair is yet another AT technique that has been developed to overcome the constraint imposed upon the waveform selection and/or design technique. This technique has an additional advantage in terms of the sidelobe reduction of the auto-correlation function (ACF) for systems utilizing noise-like pseudorandom binary sequences (PRBS, also called PN or m-sequences). PRBS waveforms offer certain performance advantages over other deterministic waveforms. These include low probability of detection (LPD) and intercept (LPI), better immunity to external electromagnetic interference (EMI), improved spectral efficiency, and immunity to jamming. A pictorial representation of a waveform-filter pair is illustrated in FIG. 1. In that technique, an input digital binary code is transformed into a new code via a matrix transformation A. This matrix acts on some features of the original code, as the sidelobe level noise, so it can be seen as a filter. The resulting output is a new code, called reference code. In a typical radar system, the use of PRBS, however, gives rise to high correlation sidelobes (SL) also called code self noise. SL is a well known problem in radar and communication systems that use binary sequences. The code self-noise makes it harder to detect the weaker echoes from smaller targets, thereby limiting the dynamic range of the radar system utilizing such waveforms. Different techniques have been proposed to minimize the SL level; these include windowing, coding, mismatched filtering, and others. Overall system performance can attain real-time enhanced detection performance by dynamically adapting the parameters of the transmitted waveforms such that SL reduction is achieved. Adaptive SL mitigation technique is however linked to the length M of the sequence. Although, varying the sequence length M to reduce the SL level can be seen as an effective AT technique, subject to hardware constraints, it usually works in trade-off with the spectral purity of the transmitted waveform. In general, binary sequences have significant out-of-band spectral leakage and are not spectrally clean or fully band-limited. Spectrally clean transmitted waveform is, however, an important design issue, if emission control levels must be respected. It also influences other system operational features, such as LPD and LPI.
There is thus a need for a method and system which simultaneously provides low sidelobe level and spectral purity without degrading the main peak of the auto-correlation function.